The life span is thought to be biologically fixed for each species, and the length of the human life span is uncertain, but may be up to 120 years. Since life expectancy has risen significantly in this century, the elderly are an increasing segment of our population, and their health care needs will continue to grow for decades.
Bio-available metal ions play crucial roles in a number of important biological processes. It is estimated that one-third of all proteins are metalloproteins (proteins containing a tightly bound metal ion) and therefore a number of biologically important processes are impaired if bio-available metal levels are either elevated or suppressed. In addition, even if there are adequate levels of bio-available metal in a biological system it is important that its distribution in the biological system be such that the biological processes that rely on the presence of the metal function appropriately.
Whilst there is a wide range of ways in which bio-available metals impact on biological systems, two of the better known would be the role of metals in enzyme systems and the role of metals in signaling mechanisms within biological systems. Examples of the role of metals in biological processes include the potential importance of Zn in the β-amyloid plaques of Alzheimer's disease; the effect of the (Cu, Zn) superoxide dismutase enzyme in mediating reactive oxygen species damage associated with amyotrophic lateral sclerosis; the participation of the heme enzymes NO synthase and guanylyl cyclase in the production and sensing, respectively, of nitric oxide (NO), and the discovery of a “zinc-finger” motif in the breast and ovarian cancer susceptibility gene, BRCA1 merely by way of example. It is also known that Cu plays a role in XIAP activity which modulates caspase activity which in turn controls apoptosis. Apoptosis is a process of controlled cell death and dysregulation of this process has been implicated in many disease states.
A large percentage of newly discovered enzymes and proteins also contain metal ions at their active sites and variations in metal levels can significantly interfere with the functioning of these enzymes and proteins. Metalloenzymes of this type are involved in a number of important bio catalytic processes including reduction of excess oxygen species. Accordingly, whenever there is either too high or too low a level of metals present in a biological system either too high a level or too low a level the normal biological processes are interrupted, typically leading to undesirable consequences. This typically occurs as many of the crucial enzymatic processes that provide protection in the biological system are suppressed or inactivated leading to undesirable consequences.
As a result of the importance of metals in the biological environment, research conducted into the roles of metals in biological systems have identified a number of conditions which are caused by or associated with abnormal levels of metal in the biological environment. In respect of these conditions they are all typically ones in which metal delivery can prevent, alleviate or ameliorate the condition. An example of a condition of this type is oxidative stress which is related to abnormal metal levels as many of the protective enzymes responsible for alleviating oxidative stress are deactivated if biological metal levels are too low.
Research in the last few decades has identified that there are a number of conditions that are caused by or associated with oxidative stress placed on the body. For example a number of cardiovascular conditions have been identified that are the result of oxidative stress (OS). Other conditions associated with OS include cancer, cataracts, neurodegenerative disorders such as Alzheimer's disease and heart diseases. In addition, there's evidence that OS plays a prominent role in three types of neuromuscular disorders: amyotrophic lateral sclerosis (ALS), mitochondrial/metabolic disease and Friedreich's ataxia.
The effect of OS is not limited to any one part of the human body, with examples of the negative effects of OS being observed for almost all organs. For example, the human brain is an organ that concentrates metal ions and recent evidence suggests that a breakdown in metal homeostasis plays a critical role in a variety of age-related neurodegenerative diseases. Common features of these diseases include the deposition of misfolded protein (each disease can have its own specific amyloid protein) and substantial cellular damage as a result of OS. Significant data suggests that OS is the primary cause of physical damage in a wide range of disease states, including amyloidogenic neurological disorders such as Alzheimer's disease (AD), amylotrophic lateral sclerosis (ALS), prion diseases—including Creutzfeldt-Jakob Disease (CJD), transmissible spongioform encephalopathies (TSE), cataracts, mitochondrial disorders, Menke's disease, Parkinson's disease (PD) and Huntington's disease (HD). [Bush, 2000 (Curr Opin Chem Biol. 2000 April; 4(2):184-91)].
In this regard, it is notable that Copper metal ion deficiency has been reported as a condition associated with AD. Copper is an essential element that is required for many enzymes to function properly, particularly those enzymes that maintain a balance in antioxidant/pro-oxidant homeostasis such as superoxide dismutase and cytochrome C oxidase. One consequence of copper deficiency is that the protective enzymes responsible for detoxifying reactive oxygen species (ROS) are inadequately loaded with copper and therefore do not effectively carry out normal enzyme function. The inadequate loading of such protective enzymes, for example in the brain, leads to a general increase in OS (as is observed in AD) which will be reflected in increased protein oxidation, such as increased protein carbonyls.
A number of therapeutic agents have been developed in an attempt to provide therapeutic solutions to the conditions caused by or associated with OS as discussed above with varied results. In general, in order to lower OS levels, various antioxidants are being used. The most common are vitamin E and vitamin C. However, vitamin E was found to be ineffective at decreasing the oxidative stress at the substantia nigra (The Parkinson Study Group, 1993, Offen et al., 1996) since this compound, although capable of crossing the blood brain barrier, is trapped in the cell membrane and therefore does not reach the cytoplasm where its antioxidant properties are needed. Vitamin C also does not cross the blood brain barrier and therefore, cannot be used effectively for neurodegenerative diseases of central origin.
There is thus still a need for, and it would be highly advantageous to have novel antioxidant compounds and methods for use of antioxidants in treatment of disease associated with oxidative damage, central nervous system neurodegenerative disorders such as PD, AD and CJD, and neurodegenerative disorders such as multiple sclerosis.
Treatment is further desirable for and in treating conditions of peripheral tissues, such as acute respiratory distress syndrome, ALS, atherosclerotic cardiovascular disease and multiple organ dysfunction. During such treatment, the complexes can act as oxygen scavengers to lower the OS within and in the vicinity of affected cells and this treatment eventually stops cell death which is associated with OS in the brain and/or peripheral tissues.
The present invention is therefore based on the finding that certain metal complexes are effective in delivering bio-available metal and could thus be used in the treatment of conditions which can be prevented, treated or ameliorated by metal delivery. In certain conditions, it is desirable that the metal be released in the cell such that after metal delivery the metal is present in the form of the free cation and it is the free cation that leads to the observed biological activity. In respect of other conditions, it is desirable that the metal stay in the form of the bound complex even after metal delivery and with these conditions it is the bound form of the metal (the metal complex) that is biologically active in the cell.
In particular, these complexes were found to be effective in delivering metal to the cells in a form which lead to a significant anti-oxidant effect being observed in the cell. Thus, certain metal complexes demonstrated an ability to mediate OS.
All references, including any patents or patent applications, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country.